Circulating precursors of human CD1c+ and CD141+ dendritic cells

Two subsets of conventional dendritic cells (cDCs) with distinct cell surface markers and functions exist in mouse and human. The two subsets of cDCs are specialized antigen-presenting cells that initiate T cell immunity and tolerance. In the mouse, a migratory cDC precursor (pre-CDC) originates from defined progenitors in the bone marrow (BM). Small numbers of short-lived pre-CDCs travel through the blood and replace cDCs in the peripheral organs, maintaining homeostasis of the highly dynamic cDC pool. However, the identity and distribution of the immediate precursor to human cDCs has not been defined. Using a tissue culture system that supports the development of human DCs, we identify a migratory precursor (hpre-CDC) that exists in human cord blood, BM, blood, and peripheral lymphoid organs. hpre-CDCs differ from premonocytes that are restricted to the BM. In contrast to earlier progenitors with greater developmental potential, the hpre-CDC is restricted to producing CD1c⁺ and CD141⁺ Clec9a⁺ cDCs. Studies in human volunteers demonstrate that hpre-CDCs are a dynamic population that increases in response to levels of circulating Flt3L.Conventional DCs (cDCs) induce immunity or tolerance by capturing, processing, and presenting antigen to T lymphocytes (Banchereau and Steinman, 1998). In the mouse, cDCs are short-lived cells, whose homeostasis in lymphoid and nonlymphoid tissues is critically dependent on continual replenishment from circulating pre-CDC (Liu et al., 2007; Liu and Nussenzweig, 2010). Murine pre-CDCs are BM-derived cells that are present in very small numbers in the blood but increase in response to Flt3L injection (Liu et al., 2007, 2009). pre-CDCs have a very short dwell time in the blood, 65% of these cells leave the circulation within 1 min after leaving the BM (Liu et al., 2007, 2009). Upon leaving the circulation, pre-CDCs seed tissues where they differentiate to cDCs, which divide further under the control of Flt3L (Liu et al., 2007, 2009). Thus, in addition to the BM and blood, mouse pre-CDCs are also found in peripheral lymphoid organs and nonlymphoid tissues (Naik et al., 2006; Bogunovic et al., 2009; Ginhoux et al., 2009; Liu et al., 2009; Varol et al., 2009).Mouse cDCs can be divided into two major subsets, CD11b⁺ DCs and CD8⁺/CD103⁺ DCs that differ in their microanatomic localization, cell surface antigen expression, antigen-processing activity, and ability to contribute to immune responses to specific pathogens (Merad et al., 2013; Murphy, 2013). Despite these important differences, both CD11b⁺ and CD8⁺/CD103⁺ cDC subsets of mouse DCs are derived from the same immediate precursor (pre-CDC) that expresses CD135 (Flt3), the receptor for Flt3L, a cytokine that is critical to DC development in vivo (McKenna et al., 2000; Waskow et al., 2008).Similar to the mouse, humans have two major subsets of cDCs. CD141 (BDCA3)⁺Clec9a⁺ DCs (CD141⁺ cDC herein) appear to be the human counterpart of mouse CD8⁺/CD103⁺ DCs, expressing XCR1, Clec9a, IRF8, and TLR3 and producing IL-12 (Robbins et al., 2008; Bachem et al., 2010; Crozat et al., 2010; Jongbloed et al., 2010; Poulin et al., 2010; Haniffa et al., 2012). CD1c (BDCA1)⁺ cDCs appear to be more closely related to mouse CD11b⁺ DCs, expressing IRF4, inducing Th17 differentiation upon A. fumigatus challenge, and imprinting intraepithelial homing of T cells (Robbins et al., 2008; Crozat et al., 2010; Schlitzer et al., 2013; Yu et al., 2013). In the mouse, the superior ability of CD8⁺/CD103⁺ DCs to cross-present exogenous antigens to CD8⁺ T cells is attributed to both differential antigen uptake (Kamphorst et al., 2010) and to increased expression of proteins and enzymes that facilitate MHC class I presentation (Dudziak et al., 2007). Human CD141⁺ cDCs are more efficient than CD1c⁺ cDCs in cross-presentation (Bachem et al., 2010; Crozat et al., 2010; Jongbloed et al., 2010; Poulin et al., 2010), but this difference appears to result from differences in antigen uptake and cytokine activation rather than a specialized cell-intrinsic program (Segura et al., 2012; Cohn et al., 2013; Nizzoli et al., 2013).Both CD1c⁺ cDCs and CD141⁺ cDCs are present in human blood and peripheral tissues. Each subset in the blood resembles its tissue counterpart in gene expression but appears less differentiated (Haniffa et al., 2012; Segura et al., 2012; Schlitzer et al., 2013). These observations are consistent with the idea that less differentiated human cDCs travel through the blood to replenish the cDC pool in the peripheral tissues (Collin et al., 2011; Segura et al., 2012; Haniffa et al., 2013). Others have postulated the existence of a less differentiated circulating DC progenitor based on absence of CD11c, expression of CD123, and response to Flt3L (O’Doherty et al., 1994; Pulendran et al., 2000), but the progenitor potential of these putative precursors that produced large amounts of IFN-α was never tested directly and they appear to correspond at least in part to plasmacytoid DCs (Grouard et al., 1997; Siegal et al., 1999). Thus, whether there is an immediate circulating precursor restricted to human immature and mature CD1c⁺ and CD141⁺ cDCs is not known.Here, we report the existence of a migratory pre-CDC in humans (hpre-CDC) that develops from committed DC progenitors (hCDPs) in the BM (Lee et al., 2015) and is the immediate precursor of both CD1c⁺ and CD141⁺ cDCs, but not pDCs or monocytes. hpre-CDCs are present in BM, cord, and peripheral blood, as well as peripheral lymphoid tissues. In studies of human volunteers, Flt3L injection induces expansion of hpre-CDCs in the circulation. Thus, human cDC precursors constitute a dynamic circulating population whose homeostasis is regulated by Flt3L, a cytokine that is responsive to inflammatory and infectious agents.